Introduction to Atoms
Explore the building blocks of all matter — atoms — including their structure, how elements are defined, and the basics of the periodic table.
What is an Atom?
Everything around you — air, water, your desk, your body — is made of matter. All matter is made up of extremely tiny particles called atoms. Atoms are so small that a single human hair is about one million atoms wide!
The word "atom" comes from the Greek word atomos, meaning "indivisible." While we now know atoms can be split, they are still considered the basic unit of an element.
Australian Curriculum Connection
This lesson aligns with AC9S7U03: "All matter is made of atoms which are composed of protons, neutrons and electrons; natural and man-made materials have different properties."
Structure of an Atom
Every atom has two main regions: a central nucleus and surrounding electron shells.
Atomic Structure — Carbon Atom (6 protons, 6 neutrons, 6 electrons)
Proton
Found in nucleus. Positively charged (+1). The number of protons defines the element (atomic number).
Neutron
Found in nucleus. No charge (neutral). Adds mass to the atom without changing its element.
Electron
Orbit the nucleus in shells. Negatively charged (−1). Involved in chemical bonding.
Elements and the Periodic Table
An element is a pure substance made of only one type of atom. Elements cannot be broken down into simpler substances by chemical means. There are 118 known elements, organised in the periodic table.
Reading an Element on the Periodic Table
| Element | Symbol | Atomic Number | Found in |
|---|---|---|---|
| Hydrogen | H | 1 | Water, fuels, stars |
| Carbon | C | 6 | All living things, coal, diamonds |
| Oxygen | O | 8 | Air, water, rocks |
| Iron | Fe | 26 | Steel, blood (haemoglobin), Earth's core |
| Gold | Au | 79 | Jewellery, electronics, Australian reserves |
Key Vocabulary
Atom
The smallest particle of an element that retains the chemical properties of that element.
Atomic Number
The number of protons in the nucleus of an atom; this number uniquely identifies each element.
Element
A pure substance made of only one type of atom; cannot be broken down by chemical means.
Nucleus
The dense, central core of an atom containing protons and neutrons.
Worked Examples
How many protons, neutrons, and electrons does an oxygen atom have?
Oxygen has atomic number 8 and mass number 16.
Protons: = atomic number = 8
Electrons: = protons (neutral atom) = 8
Neutrons: = mass number − atomic number = 16 − 8 = 8
Identify the element with 26 protons.
Step 1: The atomic number equals the number of protons. Atomic number = 26.
Step 2: Look up atomic number 26 on the periodic table.
Answer: The element is Iron (Fe).
Why is a carbon atom different from a nitrogen atom?
Carbon: atomic number 6 → 6 protons in nucleus.
Nitrogen: atomic number 7 → 7 protons in nucleus.
Answer: Carbon and nitrogen are different elements because they have different numbers of protons. The number of protons (atomic number) defines which element an atom is.
Knowledge Check
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Key Concepts Summary
- ✓ All matter is made of atoms — the smallest unit of an element.
- ✓ Atoms contain protons (+), neutrons (neutral) in the nucleus, and electrons (−) in shells around the nucleus.
- ✓ The atomic number = number of protons, and uniquely identifies each element.
- ✓ Neutrons = mass number − atomic number.
- ✓ The periodic table organises all 118 known elements by atomic number.